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High Slump Concrete 6

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Stenbrook

Stenbrook

Structural
Dec 5, 2014
125
I have a contractor who has had some issues on a previous job with the concrete showing up at the site with a higher slump then specified and allowed the pour to continue anyways. On this previous job, the concrete strengths came out low consistently across many of the pours and caused a bunch of issues. We are now in the construction phase of a new job that is very similar to this one with all of the same players (engineer, architect, owner, contractor, and concrete provider). This contractor wants to ensure that the concrete poured is acceptable and has asked us (the EOR) to allow them to add a bag of cement to the concrete mix if they should encounter a high slump while pouring.

My first thought is that this is a bad idea as it changes the mix design that the concrete provider is providing. I would assume that once you alter the mix, the provider may no longer have any liability for the concrete and it would fall on the contractors (or engineers) shoulders. At the end of the day I believe that adding some cement would probably only help the mix, but I don't want to be the one responsible for the concrete when the provider should be providing concrete with the correct slump in the first place.

Does anyone have any thoughts on this? Is it acceptable to add cement to a concrete truck with a high slump? Has anyone run into this issue before? Any input would be helpful.

Thanks in advance!
 
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High slump concrete with weak concrete indicates to me you should probably be sending the trucks back. Let the contractor know you won't accept this and they should be stricter about QC with their supplier.
 
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@ oldestguy, I had found that article but wanted to see about real world experience as well. The contractor was mainly concerned with the fact that he takes the slump test at the middle of the truck so they are already pouring so by the time they find out that the slump is high, it is too late.
 
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High slump is not normally good... the truck should be allowed to mix the concrete until the maximum time for transit and if still outside slump range, it should be rejected.

Added: Slump test sampling should be as per usual...

Dik
 
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have the inspector check the batch ticket for water and figure out w/c ratio. I imagine its a problem with that. If w/c ratio is normal but with high slump, then admixtures were likely used but strength should not be a big concern. Was w/c ratio OK in the previous project and still low strength?
 
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I don't recall them measuring the w/c ratio at the field so I am unsure if it was ok when they poured.
 
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In that case the extra-sack-of-cement fix won't solve the problem either. This is on the contractor and his concrete supplier to fix.

The guys placing the concrete can probably call the slump when it drops from the chute. If it is suspect, have the sampler pull a slump then and send that truck down the road if it isn't in spec.
 
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High slump can cause excessive shrinkage, difficulty finishing and bleedwater issues... not good.

Dik
 
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I would have the contractor address this with the concrete supplier in advance. You can be part of this conversation too if you want. I would give the supplier fore-warning that you will be sending trucks back if they do not meet the slump requirements.

Additionally, if the job is large enough, request that the concrete supplier's salesperson be on site for the first few trucks to make sure everything gets off to smooth start.

 
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To protect yourself, as EOR, from arguments and possible free services, You should write a letter to the GC, sub contractor, and those who relate that any trucks with concrete that does not meet the specified requirements will be rejected immediately, concrete placed that has issues due to poor mix will be the contractor and supplier responsibility to correct at no cost to owner or EOR or Inspectors.

Something like that will put them on notice. There will be phone calls and emails but in the end they will step up by providing the right concrete.

Don't allow field modifications except those directed by the supplier (and done by the supplier!) at the time of issue. If their immediate fix doesn't correct the issue by batch-to-placement times then the truck is rejected. 90 mins don't reset/start again after they mess with mix... I've had that argument before :)
 
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Its the responsibility of the concrete supplier to run the numbers using the increased amount of cement and determine the resulting design strength. I don't necessarily agree that adding cement will offset the effect of a high slump. It sounds to me that the assumption is that the high slump is due to extra water. Assuming that's true, adding cement will help the mix by evening out the w/c ratio. However, I see 3 issues. First, the high slump could be due to incorrect amount of plasticizers being added. Second, just adding cement will not give you the same mix design. You also need to add the fine aggregate and coarse aggregate proportionally also in order to achieve the original mix design. Third, even if adding cement is the right solution, how do you know that a bag of cement is the correct amount to add?
 
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No. No. No! Do not modify the mix design. That is the purview of the ready-mix supplier. Keep them responsible for the mix design.

Any modifications of the mix design in the field should be done by the QC representative of the supplier.

If you are having low strengths you need to understand why. Is it because your finishers are adding water at the site? Is it because the supplier is not providing the specified slump at delivery?

Low strengths are usually a result of adulteration of the mix design. Adding cement without consideration of other mix properties will increase shrinkage and will likely present placement issues.
 
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Listen to Ron. Slump is only an indicator of consistency, not necessarily of strength. Do not allow the contractor to add anything at site. A few rejected trucks should get the attention of the supplier.
 
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I agree with the previous posts. Don't approve anything, this isn't your problem to get involved with or to solve.

There are a lot of people saying that the trucks should be rejected. In my experience, it is inappropriate for an inspector to approve or reject anything. They are simply there to observe what it being built. So to the folks calling for rejected trucks - are you suggesting that the EOR or a representative from their office be physically present on site to observe the slump results and reject trucks? Is it appropriate for an EOR to reject a truck if the slump is 1/2" too high? Granted it wouldn't be nice, but I`m not sure I understand where there is language present to justify this course of action.

Honestly, it seems to me like they're going to pour the concrete they're going to pour. You can take a hard stance, make it well known to the supplier, contractor, and owner that they need to step up their game. After that, make sure everyone is aware that concrete that doesn't meet strength or is excessively cracked will need to be removed and replaced at the contractor's expense (including the additional testing, observations, any applicable design time/meetings, etc).

 
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Once20036,

Yes, that may be the old fashioned way, but when I have rejected trucks, it worked. The modern trend of separating design from construction monitoring doesn't seem to be working very well.

It is incumbent on the designer to provide language that allows him to reject faulty material at any stage of construction, including before it is installed. Not an inspector, but certainly the engineer.

1/2" is not a big issue with slump. As I said before, slump is only a useful indicator of consistency. Nothing more. But if you want 4" slump, and the first truck arrives with 7", don't dilly around. Solve the problem before it gets out of hand.

 
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I have told the contractor that nothing is to be added to the concrete mix. I will say that this whole "can we add a bag of cement" is the contractor trying to CYA in case the concrete comes in bad. His explanation is that he can't turn the truck around because they don't take the slump test until the truck is half poured so what is he supposed to do, tell them to pick up the concrete they already poured?? He is just looking for ways to fix any issues beforehand because of the issues we had previously. In my opinion, the concrete supplier did a really bad job on the last job(I don't know how you have issues getting to 3,000psi strength when it is probably the most common mix out there), and the GC should have used someone else for this job but for some reason they didn't. Either way, thank you all for your input.
 
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Instruct them to check the slump at the beginning of the load, not the middle. And if the slump is too high, you should reject it rather than leaving it to the contractor.

Do you still use 3000 psi concrete? Where I am, that is not considered to be structural grade concrete. If they can't even provide that, your quality is severely compromised.
 
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The language I would use is "concrete is not to spec and is therefore rejected as not per the contract documents." Tell the contractor to carefully document where he intends to place that concrete because you don't want him to have to demolish anymore of the placed concrete than he has to. That should be clear enough and it allows the contractor to assume his full responsibility as the construction coordinator (there might be a valid reason that he must use the concrete? Maybe practice his concrete eraser skills?). As for the slump test, the initial slump test (and air) is for the contractor so that he doesn't place the garbage. The middle of the batch and the in-situ air test is the one we want as engineers.
 
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